8, Q-in-Q provider bridges are an excellent method for connecting
ISP subscribers to their routers. In addition, other services besides routers, e.g., IP
television sources, can be attached to the Up VLANs as well. Depending on the scaling
capabilities of the bridges and routers, their relative numbers may be different from
that shown in the figure.
Note that this model does not have a place for PPP over Ethernet. Ethernet is inherently
more reliable than the modem lines for which PPP was developed. As long as the
network uses Ethernet from the subscriber to the router, and as long as the bridges employed
follow the recommendations of IEEE 802.1 with regard to handing the frames??™
hardware checksums, PPP adds more complexity and expense than it helps.
Ongoing Developments
IEEE Project 802.1ah is, as of this writing, still under development in IEEE 802.1.
However, most of the current technical draft of that document is stable, hence its inclusion
in the ???existing??? parts of this chapter. References have also been made in this
chapter to other ongoing standards work. The four major items that may affect the
provision of Carrier Ethernet services by bridges are IEEE Project P802.
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